Oceanic crust

Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima. It is thinner than continental crust, or sial, generally less than 10 kilometers thick, however it is more dense, having a mean density of about 3.3 grams per cubic centimeter.

Composition

Although a complete section of oceanic crust has not yet been drilled, there are estimations of composition based on analyses of ophiolites, comparison of seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging (especially from ridge crests and fracture zones) and drilling. Oceanic crust is significantly simpler than continental crust and generally it can be divided in three layers.

Layer 1 consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge. Near the continental margins sediment is terrigenous, meaning derived from the land, unlike deep sea sediments which are made of tiny shells of marine organisams, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous sediments transported by turbidity currents.

Layer 2 could be divided into two parts: layer 2A – 0.5 km thick uppermost volcanic layer of glassy to finely crystaline basalt usually in form of pillow basalt, and layer 2B – 1.5 km thick layer composed of diabasedikes.

Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros and cumulateultramafic rocks. It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.

Geochemistry

The most voluminous eruptive rocks composing ocean floor are mid-ocean ridge basalts (MORBs), which are derived from low-potassium tholeiitic magmas. These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements (Th, U, Nb, Ta and Pb). There can be found MORBs enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands, the Azores and Iceland.

Life cycle

Oceanic crust is continuously being created at mid-ocean ridges. At these ridges, magma rises into the upper mantle and crust, as the plates diverge. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and is progressively older away from the ridges.

The oceanic lithosphere subducts at what are known as convergent boundaries. These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old. Plate tectonics is the study of these processes

The overall process of repeated cycles of creation and destruction of oceanic crust is known as the Wilson cycle.

Magnetic lines

The oceanic crust displays an interesting pattern of parallel magnetic lines, parallel to the ocean ridges, frozen in the basalt. In the 1950’s, scientists mapped the magnetic field generated by rocks on the ocean floor. They noticed a symmetrical pattern of positive and negative magnetic lines as they moved along the ocean floor, and the line of symmetry was at the mid ocean ridge. The fact that the anomalies were symmetrical at the mid-ocean ridge was explained by the hypothesis that new rock was being formed by magma at the mid-ocean ridges, and the ocean floor was spreading out from this point. When the magma cooled to form rock, it aligned itself with the current position of the north magnetic pole of the Earth (which has reversed many times in its past) at the time of its cooling. New magma forced the older cooled magma away from the ridge. Approximately half of the new rock was formed on one side of the ridge and half on the other.